Miniature plugs have been found to be economical and very useful for quickly and easily connecting together the telephone handset to its base and the base to the wall outlet or jack. However, a serious weakness has been found in particular with the plugs on handset cords that are pulled and flexed during normal use. This causes the plugs to move in the jacks at both the handset and the base ends of the connecting cord.
It has been demonstrated that this motion, even though it is very slight, causes, fretting, galling and pitting and erosion of the electrical contacts, and this leads to premature failure due to the electrical noise introduced into the circuit by the deteriorating contacts.
The failure mechanism is generally accepted to be as follows: The current flowing through the mated contacts is concentrated in a few small areas of high points on the contacting surfaces, even though the surfaces may be highly polished. The motion between these mated surfaces causes the current paths to be broken as the points are moved apart and the current is picked up at other high points which come into contact. The mechanical rubbing of the surfaces plus the electrical arcing which occurs as the current is broken at one point and picked up at another combine to cause rapid erosion of the contact surfaces. The arcing causes points to become welded together plus it causes metal vapors to be formed as the surface is burned away by the heat of the arc. The welded points are torn apart by the motion of the contact, thus leaving rough edges and metal fragments.
The arcing also polymerizes organic vapors that are normally found in the air, and these condense to further contaminate the contact surfaces with insulating films and particles. This debris of metal oxides, metal particles, organic material and combinations thereof accumulates between and around the contacting surfaces, thus interrupting the flow of current as the contacts are moved, and this generates electrical noise in the communication circuit. This noise can become so great that the instrument becomes useless.
Special precious metal platings, lubricants and the like have been used to delay the onset of moving contact failure; however, the only positive way to prevent this type of contact failure is to eliminate the motion between the contacting surfaces.
It is the purpose of this invention to prevent motion between the plug and jack contacts due to the forces from the cord while the phone is in use, and yet allow the plugs to be inserted and removed from the jacks without tools or special skills and to maintain the desired features of the plugs and jacks.
The invention uses a new plug which is of the miniature type used on telephone handset retractile cords. This new plug has "V" shaped ridge protrusions placed at strategic points around the periphery of the rear end of the plug; the rear end being the end where the cord enters the plug and the end which is last to enter the jack.
When the plug is pushed into the jack, these ridges are forced against the surrounding surfaces of the jack where the ridges lock the plug against side-to-side and up-and-down motion in the jack, but they do not prevent the plug from being pushed into or removed from the jack. Once the plug is inserted into the jack and then pulled back against the latch stop, there are only pulling forces exerted on the plug from the cord and therefore there is no tendency for the plug to be moved back and forth in the jack along their common axis. It is the side-to-side and up-and-down motions that must be contended with, and this is effectively done with the ridges on the plug.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.